Resistor and method for manufacturing the same

ABSTRACT

A resistor for use in high density printed circuit board, having low current noise and improved resistance accuracy, and a method of manufacturing the resistor. A resistor of the present invention includes a substrate, a pair of upper-surface electrode layers formed on the end sections of the upper surface of said substrate, a resistor layer formed so that the layer is connected electrically to said upper-surface electrode layers, a first trimming groove formed by cutting said resistor layer, a resistance restoring layer which is formed to cover at least said first trimming groove, a second trimming groove formed by cutting the resistance layer and resistance restoring layer, and a protective layer provided to cover at least the resistance layer and second trimming groove. In this way, the resistors having a superior property in both the current noise characteristic and the resistance accuracy are obtained.

THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCTINTERNATIONAL APPLICATION PCT/JP98/03051.

TECHNICAL FIELD

The present invention relates to a resistor used for high-density wiringcircuits, and a method of manufacturing the resistor.

BACKGROUND ART

One known resistor of the same category is disclosed in the JapaneseLaid-open Patent publication No. H4-102302.

The conventional resistor and a method of manufacturing the resistor aredescribed in the following with reference to drawings.

FIG. 8 is a sectional view of the conventional resistor.

In FIG. 8, first upper-surface electrode layers 2 are provided on theright and the left ends of the upper surface of the insulating substrate1; a resistor layer 3 is provided partially overlapping on the firstupper-surface electrode layers 2; a first protective layer 4 is providedto cover only the whole surface of the resistance layer 3; a trimminggroove 5 for correcting the resistance is provided by cutting throughthe resistor layer 3 and the first protective layer 4; a secondprotective layer 6 is provided to cover only the upper surface of thefirst protective layer 4; second upper-surface electrode layers 7 areprovided on the upper surface of the first upper-surface electrodelayers 2 so as to spread until the end in the width of the insulatingsubstrate 1; side electrode layers 8 are provided on the side surfacesof the insulating substrate 1; nickel plated layers 9 and solder platedlayers 10 are provided on the surfaces of the second upper-surfaceelectrode layers 7 and the side electrode layers 8.

A method of manufacturing the resistor as configured above is describednext, referring to drawings.

FIG. 9 illustrates process steps of manufacturing the conventionalresistor.

In the first place, as shown in FIG. 9(a), first upper-surface electrodelayers 2 are formed on the right and the left ends of upper surface ofthe insulating substrate 1, using a printing process.

Then, as shown in FIG. 9(b), a resistor layer 3 is formed by a printingprocess on the upper surface of the insulating substrate 1 so that partof the resistor layer overlaps on the first upper-surface electrodelayers 2.

As shown in FIG. 9(c), a first protective layer 4 is formed by aprinting process covering only the whole surface of the resistor layer3, and then a trimming groove 5 is formed by cutting through theresistor layer 3 and the first protective layer 4 using a laser, orother means, in order to adjust the overall resistance of the resistancelayer 3 to be falling within a certain predetermined range.

A second protective layer 6 is formed by a printing process coveringonly the upper surface of the first protective layer 4, as shown in FIG.9(d).

As shown in FIG. 9(e), a second upper-surface electrode layer 7 isformed on the upper surface of the first upper-surface electrode layer 2by a printing process so that the electrode layer stretches to the endsof the insulating substrate 1.

As shown in FIG. 9(f), a side electrode layer 8 is formed by a coatingprocess covering the right and the left side end surfaces of the firstupper-surface electrode layer 2 and the insulating substrate 1,electrically coupling with the first and the second upper-surfaceelectrode layers 2 and 7.

Finally, surfaces of the second upper-surface electrode layer 7 and theside electrode layer 8 are plated with nickel, and then with solder, forforming a nickel plated layer 9 and a solder plated layer 10. Theconventional resistors are manufactured through the above describedprocess steps.

However, with the conventional resistors having the above describedconfiguration and manufactured through the conventional procedure, wherea trimming groove 5 has been formed by cutting the resistance layer 3and the first protective layer 4 with a laser or other means to improvethe resistance accuracy, a current noise is generated in the resistor.

Now, the mechanism of current noise generation is described in thefollowing with reference to drawings.

FIG. 10(a) shows a relationship between the resistance correction ratioand the current noise, exhibited by a 1005 size, 10 kΩ resistor havingthe conventional configuration, manufactured through the conventionalprocess. The graph indicates that the current noise characteristic getsworse along with an increasing ratio of the resistance correction.Basically, an increased ratio of the resistance correction results in areduction in the effective resistance area of the resistor layer, whicheventually leads to a ski deteriorated current noise characteristic. Inreality, however, extent of the deterioration in the current noisecharacteristic is more than what the basic principle explains. Theresistor layer is damaged by the heat generated during the resistancecorrection in the area around the trimming groove, and by the microcracks caused thereby. The wide dispersion of the current noise startedafter the resistance correction, as shown in FIG. 10(a), represents adispersion existing in the extent of deterioration of the resistancelayer.

FIGS. 10(b), (c) show shift of the current noise generated in theresistor layer measured after the respective process steps;

FIG. 10(b) represents a resistor whose second protective layer is formedof a resin, FIG. 10(c) represents a resistor whose second protectivelayer is formed of a glass. The deterioration of current noisecharacteristic stems from the trimming process, as described earlier. Ina resistor having second resin protective layer, the deterioratedcurrent noise characteristic remains as it is until the stage offinished resistor.

Whereas, in a resistor having second glass protective layer, although asufficient amount of heat that is required for restoring the resistanceis provided at the baking process for the second protective layer thedeteriorated resistor layer is hardly repaired, because the resistorlayer has been covered by the first protective layer which was alreadybaked and the glass component can not permeate into micro cracks of theresistor layer generated during the trimming operation. Namely, thecurrent noise is hardly restored.

The current noise may be restored if the baking temperature is raised toa level at which the glass component contained in the resistor layersoftens to repair the micro cracks. In this case, however, a resistanceaccuracy achieved by the trimming operation can not stay as it is untilthe stage of finished resistor.

As described in the foregoing, a problem with the conventional resistorsconfigured above and manufactured by a conventional method to provide acertain predetermined resistance is the increased current noise due tothe heat and micro cracks generated at the vicinity of the trimminggroove during the resistance correcting operation.

The present invention addresses the above problem and aims to provide aresistor, as well as the method of manufacturing, that is superior inboth the current noise characteristic and the resistance accuracy.

DISCLOSURE OF THE INVENTION

A resistor of the present invention includes

a substrate,

a pair of upper-surface electrode layers formed on the side sections ofthe upper surface of said substrate,

a resistor layer formed so that the layer is connected electrically tosaid upper-surface electrode layers,

a first trimming groove formed by cutting said resistor layer,

a resistance restoring layer which is formed to cover at least saidfirst trimming groove,

a second trimming groove formed by cutting said resistance layer andresistance restoring layer, and

a protective layer provided to cover at least said resistance layer andsecond trimming groove.

In a resistor of the above configuration, since the resistance restoringlayer has been disposed covering the first trimming groove which wasformed by cutting the resistance layer, glass component contained in theresistance restoring layer softened and melted during the bakingoperation for forming the resistance restoring layer permeates intomicro cracks generated at the first trimming operation. Thisrehabilitates the deteriorated resistor layer; as the result, thecurrent noise decreases significantly after, formation of the resistancerestoring layer, as compared with that after the first trimmingoperation. Furthermore, dispersion of the resistance, which was somewhatill-affected when the resistance restoring layer was provided, can beimproved precisely to a specified value by a fine-adjusting operationconducted at the formation of the second trimming groove by cutting theresistance layer and resistance restoring layer. Thus the resistance canbe corrected precisely while a superior current noise characteristic ismaintained up until the state of finished resistor. In this way, theresistors having a superior property in both the current noisecharacteristic and the resistance accuracy are obtained in accordancewith the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a sectional view of a resistor in a first embodiment of thepresent invention,

FIG. 1(b) is a see-through view of the resistor viewed from the above.

FIGS. 2(a)-(d) illustrate a process for manufacturing the resistor.

FIGS. 3(a)-(e) illustrate a process for manufacturing the resistor.

FIGS. 4(a) and (b) show a relationship between the current noise and theresistance accuracy in the resistor layer, after respective processsteps in the manufacturing method.

FIG. 5(a) is a sectional view of a resistor in a second embodiment ofthe present invention,

FIG. 5(b) is a see-through view of the resistor viewed from the above.

FIGS. 6(a)-(d) illustrate a process for manufacturing the resistor.

FIGS. 7(a)-(d) illustrate a process for manufacturing the resistor.

FIG. 8 is a sectional view of a conventional resistor.

FIGS. 9(a)-(f) illustrate a process for manufacturing the conventionalresistor.

FIGS. 10(a)-(c) show a relationship between the ratio of trimming forresistance correction and the current noise in the conventionalresistor.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

A resistor in a first exemplary embodiment of the present invention anda method for manufacturing the resistor are described with reference tothe drawings.

FIG. 1(a) is a sectional view of a resistor in embodiment 1 of thepresent invention, FIG. 1(b) is a see-through view of the resistor asseen from the above.

In FIG. 1, numeral 21 denotes a substrate made of alumina or the likematerial ; a pair of upper-surface electrode layers 22 is made of amixture of silver and glass, or the like material, and is formed on theend sections of the upper surface of the substrate 21; a pair ofbottom-surface electrode layers 23 is made of a mixture of silver andglass, or the like material, and is formed, depending on needs, on theend sections of the bottom surface of the substrate 21; a resistor layer24 is made of a mixture of ruthenium oxide and glass, a mixture ofsilver, palladium and glass, or the like material, and is formed on theupper surface of the substrate 21 so that the resistor layer partlyoverlaps on the upper-surface electrode layers 22 making electricalcontact; a first trimming groove 25 is formed by cutting the resistorlayer 24 with a laser, or other means, for correcting the resistance toa certain predetermined value; a resistance restoring layer 26 is madeof a borosilicate lead glass, having a softening point of 500° C.-600°C., or the like material, and is formed to cover at least the resistorlayer 24; a second trimming groove 27 is formed by cutting theresistance layer 24 with a laser, or the like means, for fine-adjustingthe resistance to a certain predetermined value; a protective layer 28is made of a borosilicate lead glass, an epoxy resin, or the likematerial, and is formed to cover at least the resistor layer 24; a sideelectrode layer 29 is made of a mixture of silver and glass, or the likematerial, and is formed, depending on needs, on the side surface of thesubstrate 21 electrically connecting the upper-surface electrode layer22 and the bottom-surface electrode layer 23; a first plated layer 30 ismade of nickel, or the like material, and is formed, depending on needs,to cover the side electrode layer 29 and the exposed portions of theupper-surface electrode layer 22 and the bottom-surface electrode layer23; a second plated layer 31 is formed, depending on needs, to cover thefirst plated layer 30.

Next, a method for manufacturing the above-configured resistor isdescribed referring to the drawings.

FIG. 2 and FIG. 3 illustrate a process for manufacturing a resistor in afirst exemplary embodiment of the present invention.

As shown in FIG. 2(a), upper-surface electrode layers 43 are formed on asheet 42 which is made of alumina, or the like material, having lateraland longitudinal dividing slits 41, with paste of a mixture of silverand glass by screen-printing across the dividing slit 41, drying andthen baking in a continuous belt furnace under a temperature profile ofabout 850° C. for about 45 minutes. Depending on needs, bottom-surfaceelectrode layers (not shown) may be formed at the same time on thebottom surface of the sheet 42 at places opposing to the upper-surfaceelectrode layers 43 by screen-printing and drying paste of a mixture ofsilver and glass.

Then, as shown in FIG. 2(b), a resistor layer 44 is formed bridging theupper-surface electrode layers 43, with paste of a mixture of rutheniumoxide and glass by screen-printing on the upper surface of the sheet 42so that it partly overlaps on the upper-surface electrode layers 43,drying and then baking in a continuous belt furnace under a temperatureprofile of about 850° C. for about 45 minutes.

As shown in FIG. 2(c), a first trimming groove 45 is formed by a laser,or the like means, in order to correct resistance of the resistor layer44 to an 85% of the resistance of a final resistance, taking intoconsideration the possible resistance shifts during process steps itundergoes before making a finished resistor.

Then, a resistance restoring layer 46 is formed, as shown in FIG. 2(d),covering the upper surface of the resistor layer 44, with paste of aborosilicate lead glass by screen-printing, drying and then baking in acontinuous belt furnace under a temperature profile of about 620° C. forabout 45 minutes.

In order to fine-adjust the resistance of resistor layer 44, a secondtrimming groove 47 is formed by a laser, or the like means, as shown inFIG. 3(a).

As shown in FIG. 3(b), a protective layer 48 is formed covering at leastthe upper surface of the resistor layer 44 (not shown in the presentillustration), with paste of a borosilicate lead glass byscreen-printing, drying and then baking in a continuous belt furnaceunder a temperature profile of about 620° C. for about 45 minutes.

The sheet 42 is divided along a dividing slit 41 so that theupper-surface electrode layer 43 is exposed at the side of thesubstrate, as shown in FIG. 3(c); and a substrate 49 of a strip-shape isprovided.

Depending on needs, a side electrode layer 50 is formed, as shown in.FIG. 3(c), on the side surface of the strip-shape substrate 49 partlyoverlapping on the upper-surface electrode layers 43, with paste of amixture of silver and glass transfer-printed by a roller, dried and thenbaked in a continuous belt furnace under a temperature profile of about620° C. for about 45 minutes.

The substrate 49 of a strip-shape is divided into pieces 51, as shown inFIG. 3(e).

Finally, depending on needs, a first plated layer (not shown) is formedwith nickel, or the like material, covering the side electrode layer 50and the exposed portions of the upper-surface electrode layer 43 and thebottom-surface electrode layer, and a second plated layer (not shown) isformed with a tin lead alloy, or the like material, covering the firstplated layer. A resistor in exemplary embodiment 1 of the presentinvention is thus manufactured.

Although a mixed material of silver and glass has been used for formingthe protective layer in a resistor of embodiment 1 of the presentinvention, an epoxy resin, a phenolic resin or the like material may beused instead for the same purpose.

Although a mixed material of silver and glass has been used for the sideelectrode layer 50 in a resistor of embodiment 1 of the presentinvention, a nickel containing phenolic resin or the like material maybe used instead for the same purpose.

Now in the following, operation and function of the above describedresistor are described referring to the drawings.

FIG. 4 shows a relationship, after respective process steps, between thecurrent noise and the resistance accuracy in a resistor layer inembodiment 1 of the present invention. FIG. 4(a) exhibits the resistorsof embodiment 1 whose protective layer, which being a key portion, isformed of a glass, while FIG. 4(b) represents the resistors whoseprotective layer is formed of a resin.

It is seen that the current noise significantly decreases afterformation of the resistance restoring layer, as compared with that afterthe first trimming process. The reason can be explained that the glasscomponent contained in the resistance restoring layer that softened andmelted during baking for the formation of resistance restoring layer haspermeated into micro cracks generated at the first trimming operation,to repair the deteriorated resistor layer.

Furthermore, the second trimming is for fine-adjusting the resistance ofa resistor to a higher accuracy with an aim to narrow the dispersion inresistance among the resistors, which dispersion could have somewhatdeteriorated as a result of formation of the resistance restoring layer.

Therefore, if the resistance was already corrected at the first trimmingprocess to be closer to a targeted value for more than 80%, ratio of theresistance correction needed at the second trimming may be not higherthan 1.3 times relative to a resistance before the second trimming.Then, a deterioration of the current noise characteristic to be causedby the second trimming will stay only nominal.

In a case where the ratio of resistance correction at the secondtrimming is higher than 1.3 times, the current noise characteristicshows a considerable deterioration, though, not so remarkable as in theconventional resistors.

Taking advantage of the above functions, the resistors in accordancewith exemplary embodiment 1 of the present invention can undergo theresistance correction processes while preserving a state of the superiorcurrent noise characteristic up until the stage of finished resistor.Thus the resistors superior in the current noise characteristic areobtained.

Regarding the resistance accuracy after the firing of the protectivelayer, the dispersion of the resistance goes slightly greater than thatof after the second trimming among those resistors whose protectivelayer is formed of a glass. Conventional resistors also exhibit more orless the same trends. However, comparing with the conventionalresistors, the dispersion is smaller among the resistors in embodiment 1of the present invention, in which the lower degree of deteriorationexisted in the resistance layer before formation of the protectivelayer. This contributes to implement a resistor that is superior also inthe resistance-value accuracy.

Further, among the resistors whose protective layer is formed of aresin, hardly any resistance shift occurs at the formation of theprotective layer, and thereafter. Therefore, the accuracy of resistanceprovided at the stage of the second trimming can be maintained as it is,and it makes itself an resistance accuracy of a finished resistor. Thusthe resistors whose protective layer is formed of a resin exhibit asuperior resistance accuracy, as compared with those resistors whoseprotective layer is formed of a glass.

The accuracy of second trimming bears decisive factor to the resistanceaccuracy of a finished resistor. Whereas, the first trimming is notrequired to be so accurate as the second trimming. Therefore, for thepurpose of obtaining a higher productivity, the bite size, whichcorresponds to the resistance layer cutting length per one laser pulse,may be made larger in the first trimming than in the second trimming.

The resistors that are provided with superior properties in both thecurrent noise characteristic and the resistance accuracy are thusobtained by taking advantage of the above described reasons.

Depending on needs, by providing the bottom-surface electrode layer andthe side electrode layer, a resistor in embodiment 1 of the presentinvention can be mounted regardless of facing(up or down) of theresistor to a printed circuit board in a stable manner.

Next in the following, the current noise and the resistance accuracy arecompared between the resistors in embodiment 1 of the present inventionand conventional resistors.

Method of Experiment

Resistors of 1005 size, 10 kΩ finished resistance, were measured andcompared with respect to the current noise and the dispersion ofresistance value; among those of conventional configuration, those inembodiment 1 of the present invention having glass protective layer andthose having resin protective layer. The current noise was measured withan Quan-tech equipment, model 1315c.

Experimental Results

Table 1 compares measured current noise and dispersion of trimmingaccuracy between the conventional resistors and those in embodiment 1 ofthe present invention.

TABLE 1 Resistors in the embodiment 1 Conventional Glass Resin resistorsprotective layer protective layer Current noise 1.8-10.5 −2.1-−0.5−1.9-0.0 (dB) Resistance 1.22 0.98 0.43 accuracy (%) Resistance accuracy= 3 × standard deviation/average resistance × 100 (%)

As seen from Table 1, the resistors in embodiment 1 of the presentinvention are provided with smaller figures both in the current noiseand the resistance accuracy, compared with the conventional resistors.

Embodiment 2

A resistor in a second exemplary embodiment of the present invention anda method for manufacturing the resistor are described with reference tothe drawings.

FIG. 5(a) is a sectional view of a resistor in embodiment 2 of thepresent invention, FIG. 5(b) is a see-through view of the resistor asseen from the above.

In FIG. 5, numeral 61 denotes a substrate made of alumina or the likematerial; a pair of upper-surface electrode layers 62 is made of amixture of silver and glass, or the like material, formed on the sideends of the upper surface of the substrate 61; a resistor layer 63 ismade of a mixture of ruthenium oxide and glass, a mixture of silver,palladium and glass, or the like material formed on the upper surface ofthe substrate 61 so that the continuous resistor layer partly overlapson the upper-surface electrode layers 62 making direct electricalcontact; a first trimming groove 64 is formed by cutting the resistorlayer 63 with a laser, or other means, for correcting the resistance toa certain predetermined value; a resistance restoring layer 65 is madeof a borosilicate lead glass, having a softening point of 500° C.-600°C., or the like material, formed to cover at least the resistance layer63; a second trimming groove 66 is formed by cutting the resistor layer63 with a laser, or the like means, for fine-adjusting the resistance toa certain predetermined value; a protective layer 67 is made of aborosilicate lead glass, an epoxy resin, or the like material, formed tocover at least the resistor layer 63; a first plated layer 68 is made ofnickel, or the like material, formed, depending on needs, to cover theexposed portion of the upper-surface electrode layer 62; a second platedlayer 69 is formed, depending on needs, to cover the first plated layer68.

Next, a method for manufacturing the above-configured resistor isdescribed referring to the drawings.

FIG. 6 and FIG. 7 illustrate a process for manufacturing a resistor in asecond exemplary embodiment of the present invention.

As shown in FIG. 6(a), upper-surface electrode layers 73 arescreen-printed on a sheet 72 made of alumina, or the like material,having lateral and longitudinal dividing slits 71, with paste of amixture of silver and glass across the dividing slit 71, dried and thenbaked in a continuous belt furnace under a temperature profile of about850° C. for about 45 minutes.

Then, as shown in FIG. 7(b), a resistor layer 74 is screen-printedelectrically bridging the upper-surface electrode layers 73, with pasteof a mixture of ruthenium oxide and glass on the upper surface of thesheet 72 so that it partly overlaps on the upper-surface electrodelayers 73, dried and then baked in a continuous belt furnace under atemperature profile of about 850° C. for about 45 minutes.

As shown in FIG. 6(c), a first trimming groove 75 is formed by a laser,or the like means, in order to correct resistance of the resistor layer74.

Then, a resistance restoring layer 76 is screen-printed, as shown inFIG. 6(d), covering the upper surface of the resistance layer 74, withpaste of a borosilicate lead glass, dried and then baked in a continuousbelt furnace under a temperature profile of about 620° C. for about 45minutes.

In order to fine-adjust the resistance of resistor layer 74, a secondtrimming groove 77 is formed by a laser, or the like means, as shown inFIG. 7(a).

As shown in FIG. 7(b), a protective layer 78 is screen-printed coveringthe upper surface of the resistor layer 74 (not shown in the presentillustration), with paste of a borosilicate lead glass, dried and thenbaked in a continuous belt furnace under a temperature profile of about620° C. for about 45 minutes.

The sheet 72 is divided along a dividing slit 71 so that theupper-surface electrode layer 73 is exposed at the side of thesubstrate, as shown in FIG. 7(c); and a substrate 79 of a strip-shape isprovided.

The substrate 79 of a strip-shape is divided into pieces 80, as shown inFIG. 7(d).

Finally, depending on needs, a first plated layer (not shown) is formedwith nickel, or the like material, covering the exposed portion of theupper-surface electrode layer 73, and a second plated layer (not shown)is formed with a tin lead alloy, or the like material, covering thefirst plated layer.

Although a mixed material of silver and glass has been used for theprotective layer in a resistor of exemplary embodiment 2 of the presentinvention, an epoxy resin, a phenol resin, or the like material may beused instead for the same purpose.

Operational principles and functions with the above configured resistorsmanufactured through the above manufacturing process remain the same asthose in embodiment 1 of the present invention. So, description on whichis omitted here. In the following, the resistors in embodiment 2 of thepresent invention and conventional resistors are compared with respectto the current noise and the resistance accuracy.

(Method of Experiment)

Resistors of 1005 size, 10 kΩ finished resistance, were measured andcompared with respect to the current noise and the dispersion ofresistance, between those of conventional configuration and those inembodiment 2 of the present invention having resin protective layer. Thecurrent noise was measured with an Quan-tech equipment, model 1315c.

Experimental Results

Table 2 compares measured current noise and dispersion of trimmingaccuracy, between the conventional resistors and those in embodiment 2of the present invention.

TABLE 2 Resistors in the embodiment 1 Conventional resin resistorsprotective layer Current noise 1.8-10.5 −2.1-−0.1 (dB) Resistance 1.220.46 accuracy (%) Resistance accuracy = 3 × standard deviation/averageresistance × 100 (%)

As seen from Table 2, the resistors in embodiment 2 of the presentinvention exhibit smaller figures in both the current noise and theresistance accuracy, compared with the conventional resistors.

INDUSTRIAL APPLICABILITY

A resistor of the present invention includes a substrate, a pair ofupper-surface electrode layers formed on the end sections of the uppersurface of said substrate, a resistor layer formed so that the layer isconnected electrically to said upper-surface electrode layers, a firsttrimming groove formed by cutting said resistance layer, a resistancerestoring layer which is formed to cover at least said first trimminggroove, a second trimming groove formed by cutting said resistor layerand resistance restoring layer, and a protective layer provided to coverat least said resistor layer and second trimming groove.

In a resistor of the above configuration, since the resistance restoringlayer has been disposed covering the first trimming groove provided bycutting the resistance layer, the glass component contained in theresistance restoring layer softened and melted during the bakingoperation for forming the resistance restoring layer permeates intomicro cracks generated at the first trimming operation. This repairs thedeteriorated resistor layer; as a result, the current noise afterformation of the resistance restoring layer shows a significant decreaseas compared with that after the first trimming operation.

Furthermore, dispersion of the resistance, which was somewhatill-affected by the formation of said resistance restoring layer, isimproved as a result of a fine-adjusting operation in which the secondtrimming groove is provided by cutting said resistance layer andresistance restoring layer in order to bring the resistance to aspecified value.

Thus, the resistance can be corrected precisely with a resistor of thepresent invention having the above described configuration, while asuperior current noise characteristic is maintained excellent until afinished resistor.

In this way, resistors that are superior both in the current noisecharacteristic and in the resistance accuracy can be obtained inaccordance with the present invention.

What is claimed is:
 1. A resistor comprising: a substrate, a pair ofupper-surface electrode layers formed on the end sections of the uppersurface of said substrate, a continuous resistor layer formed so that itis connected directly to said upper-surface electrode layers, a firsttrimming groove formed by cutting said resistance layer, a resistancerestoring layer formed to cover at least said first trimming groove, asecond trimming groove formed by cutting said resistance layer away fromsaid resistance restoring layer, and a protective layer provided tocover at least said resistance layer and second trimming groove.
 2. Theresistor of claim 1, further comprising: a pair of bottom-surfaceelectrode layers formed on the end sections of the bottom surface of thesubstrate, and side electrode layers formed on the side surfaces of thesubstrate electrically connecting the upper-surface electrode layer andsaid bottom-surface electrode layers.
 3. The resistor of claim 2,wherein the length of the first trimming groove is set for a lengthneeded to attain a resistance of not less than 80% of a targetedresistance.
 4. The resistor of claim 2, wherein the length the secondtrimming groove is set for a length by which the ratio of resistancecorrection after the second trimming is not higher than 1.3 timesrelative to the resistance before the second trimming.
 5. The resistorof claim 2, wherein the resistance restoring layer is formed of aborosilicate lead glass having a softening point of 500° C.-600° C. 6.The resistor of claim 2, wherein the protective layer is formed of anepoxy resin or a phenolic resin.
 7. The resistor of claim 1, wherein thelength of the first trimming groove is set for a length needed to attaina resistance of not less than 80% of a targeted resistance.
 8. Theresistor of claim 1, wherein the length the second trimming groove isset for a length by which the ratio of resistance correction after thesecond trimming is not higher than 1.3 times relative to the resistancebefore the second trimming.
 9. The resistor of claim 1, wherein theresistance restoring layer is formed of a borosilicate lead glass havinga softening point of 500° C.-600° C.
 10. The resistor of claim 1,wherein the protective layer is formed of an epoxy resin or a phenolicresin.
 11. The resistor of claim 1, further comprising: a pair of sideelectrode layers provided on the side surface of the substrate, whichside electrode layer being electrically connected with the upper-surfaceelectrode layer.
 12. The resistor of claim 11, wherein the length of thefirst trimming groove is set for a length needed to attain a resistanceof not less than 80% of a targeted resistance.
 13. The resistor of claim11, wherein the length the second trimming groove is set for a length bywhich the ratio of resistance correction after the second trimming isnot higher than 1.3 times relative to the resistance before the secondtrimming.
 14. The resistor of claim 11, wherein the resistance restoringlayer is formed of a borosilicate lead glass having a softening point of500° C.-600° C.
 15. The resistor of claim 11, wherein the protectivelayer is formed of an epoxy resin or a phenolic resin.
 16. The resistorof claim 1, wherein said second trimming groove is formed through saidresistor layer and said resistance restoring layer.